1. Field of the Invention
This invention relates generally to force transducers and, more particularly, to resistive strain gauge force transducers for actuators.
2. Description of the Related Art
Force transducers for control actuators convert external forces applied to the actuator by a user into corresponding electrical signals that are used to control a device or position an object of a display. The external forces are converted into electrical signals that represent force components along orthogonal axes. For example, a joystick-type display controller for a computer includes a force transducer that converts forces applied to the joystick into two signals, one signal representing the component of force along an x axis and a second signal representing the component of force along a y axis that is orthogonal to the x axis. The relative magnitude of the x and y signals represent the relative amount of display pointer movement along the x and y axis desired by the user. A computer to which the display controller is attached receives the electrical signals and moves the display pointer accordingly. The force transducer of the display controller is provided with electrical power and generates the electrical signals representing the force components as the external force is applied.
The joystick-type display controller described above can include a force transducer comprising a lever arm that is mechanically coupled to a support base by an articulated joint. While the force transducer provides the needed force component electrical signals, the mechanical linkage needed to permit movement of the joystick in the desired directions is complex. In particular, the mechanical linkage is relatively large, bulky, and expensive. Also, the mechanical linkage is subject to wear and reliability problems.
As an alternative to the joystick force transducer with a mechanical linkage, force transducers have been developed with thin film resistive strain gauges. A thin film resistive strain gauge uses a conductive, thin film resistive strain gauge material that is deposited onto a thin, flexible substrate using photolithographic fabrication techniques. The strain gauge material undergoes a change in electrical resistivity when the underlying substrate is subjected to strain. If an electrical current is passed through the strain gauge material, the change in resistivity can be detected by an output voltage change and the relative amount of strain can be measured. The substrate is bonded onto the sides of the lever arm so that the substrate undergoes strain when forces are applied to the lever arm.
For example, the International Business Machine Corporation (IBM Corporation) "ThinkPad 750" laptop computer is provided with a display controller force transducer comprising a pointer actuator that extends upwardly from between keys of the computer keyboard between the left and right hands of a computer user whose hands are resting at the keyboard home position. The lever arm includes the thin film strain gauges described above. As forces are applied to the lever arm, the strain is detected and is used to control a display pointer. This permits a user to control the associated display pointer without removing his or her hands from the keyboard. The user's hands otherwise would need to move from the keyboard to manipulate, for example, a display mouse controller or joystick controller.
The force transducer using thin film resistive strain gauge technology represents a valuable advance over mechanical linkage force transducers. The thin film force transducer is much smaller and lighter than the mechanical linkage and therefore is ideally suited to installation in tight spaces, such as beneath the keyboard of a laptop computer. Moreover, the thin film force transducer contains no moving parts. Thus, the wear and reliability problems of the mechanical linkage are virtually eliminated. Unfortunately, the thin film resistive strain gauges produce output signals with a relatively low signal-to-noise ratio. The electrical environment within a laptop computer includes many different sources of electrical and magnetic interference that make processing the force transducer output signals difficult. In addition, the thin film resistive strain gauges are relatively expensive to produce.
From the discussion above, it should be apparent that there is a need for a force transducer that is sufficiently small to be suitable for use in tight spaces such as computer keyboards, provides improved signal-to-noise characteristics for easier signal processing, is reliable for improved performance and greater service life, and can be produced at a competitive cost. The present invention satisfies this need.